Answer:
Observational Study
Explanation:
An experimental study is a research study in which conditions are under the direct manipulation of the investigator. We use it to mainly test the efficacy of a medical, preventive, or therapeutic measure.
In an observational study, on the other hand, the researcher observes the effect without trying to influence the outcome in any way.
In this study, the researchers are trying to determine the paired daytime and nighttime counts of the species of fishes made by the same divers in six lakes during September 2015.
This is simply an Observational Study as nothing is done to influence the population count.